1. Technical Field
The present disclosure relates to an organic light-emitting diode (OLED) display panel, an OLED display device including the same, and a method for driving the same. More specifically, the present disclosure relates to an OLED display panel further including a switching transistor for controlling application of supply voltage in the initializing interval of a pixel, an OLED display device including the same, and a method for driving the same.
2. Description of the Related Art
As the information-oriented society evolves, various demands for display devices are ever increasing. Recently, a variety of flat display devices such as liquid-crystal display (LCD) devices, plasma display panel (PDP) devices, and organic light-emitting diode (OLED) display devices have been utilized.
Among these, an OLED display device is advantageous over other flat display devices in that it can be driven with low voltage, can be made thinner, has good viewing angle and fast response speed, and so on. Accordingly, OLED display devices find more and more applications.
FIG. 1 is a circuit diagram of a pixel of an OLED display device in the related art, FIG. 2 is a timing chart for driving the pixel, and FIG. 3 is a graph showing response time (R/T) characteristics according to different initializing time intervals.
FIG. 1 is an equivalent circuit diagram of a pixel of an OLED display device in the related art, which has the typical 6T1C (six transistors and one capacitor) structure.
Referring to FIG. 1, the pixel of the typical OLED display device includes six transistors, one capacitor, an OLED, etc.
That is, in the pixel area, first to fourth transistors T1 to T4, a switching transistor T_sw, a driving transistor T_dr, a storage capacitor C, and an OLED may be formed.
The first to fourth transistors T1 to T4, the switching transistor T_sw and the driving transistor T_dr may be p-type transistors.
The source electrode of the switching transistor T_sw is connected to a data line, the gate electrode of the switching transistor T_sw is connected to a scan line, and the drain electrode of the switching transistor T_sw is connected to a terminal of the storage capacitor C. The switching transistor T_sw is turned on when a scan signal Scan is applied via the scan line to allow data voltage to be applied to the storage capacitor C.
The source electrode of the first transistor T1 is connected to a reference voltage line Vref and the gate electrode of the first transistor T1 is connected to an emission control line, and the drain electrode of the first transistor T1 is connected to the terminal of the storage capacitor C. The first transistor T1 is turned on when an emission control signal EM is applied via the emission control line to allow reference voltage Vref to be applied to the terminal of the storage capacitor C.
The source electrode of the second transistor T2 is connected to the other terminal of the storage capacitor C, the gate electrode of the second transistor T2 is connected to the scan line, and the drain electrode of the second transistor T2 is connected to the drain electrode of the driving transistor T_dr.
The source electrode of the third transistor T3 is connected to the drain electrode of the driving transistor T_dr, the gate electrode of the third transistor T3 is connected to the emission control line, and the drain electrode of the third transistor T3 is connected to the anode electrode of the OLED.
The source electrode of the fourth transistor T4 is connected to the anode electrode of the OLED, the gate electrode of the fourth transistor T4 is connected to the scan line, and the drain electrode of the fourth transistor T4 is connected to the reference voltage Vref line.
The source electrode of the driving transistor T_dr is connected to the supply voltage VDD_EL terminal, the gate electrode of the driving transistor T_dr is connected to the other terminal of the storage capacitor C, and the drain electrode of the driving transistor T_dr is connected to the drain electrode of the second transistor T2. While the driving transistor T_dr is turned on, current flows to the OLED so that the OLED emits light.
The intensity of the light emitted from the OLED is proportional to the amount of the current flowing in the OLED, and the amount of the current flowing in the OLED is proportional to the magnitude of the data voltage DATA applied to the gate electrode of the driving transistor T_dr.
In this manner, the OLED display device can display a variety of images by applying data voltages having different magnitudes to the pixel areas to display different gradations.
The storage capacitor C holds data voltage DATA for a frame to regulate the amount of the current flowing in the OLED and maintains the gradation displayed by the OLED.
FIG. 2 is a timing chart for driving the OLED display device of FIG. 1.
Referring to FIG. 2, it can be seen that the emission control signal EM is deactivated immediately after the scan signal Scan is applied. In doing so, data addressing and Vth (threshold voltage) compensation are carried out. In particular, the time period in which both of the emission control signal EM and the scan signal Scan are in on-state is the initializing time interval I of the pixel. It is noted that since the transistors are P type transistors, the emission control signal EM and scan signal Scan are active and in the on-state when they are logic low, and they are deactivated and in the off-state when they are logic high.
For the pixel having the 6T1C structure described above with reference to FIG. 1, all of the transistors are turned on during the initializing time interval I in which both of the emission control signal EM and the scan signal Scan are in on-state.
In other words, the gate electrodes of all of the transistors T_sw, T_dr and T1 to T4 disposed in the pixel receive the emission control signal EM or the scan signal Scan directly or indirectly, and thus all of the transistors remain turned on during the time interval I in which the scan signal is applied on the scan line, and the signal on the emission control line EM is in an on-state.
As a result, a short-circuit is created between the supply voltage VDD_EL and the reference voltage Vref during the initializing time interval I. That is, the initialization voltage applied at the gate terminal of the driving transistor T_dr equals to:VDD_EL−Vref−a, 
where a denotes a voltage that varies depending on data of a previous frame.
Due to the voltage a, the voltage at the gate terminal of the driving transistor T_dr increases in black screens while it decreases in white screens, such that deviation in the initial voltage used in sampling occurs, resulting in response time delay.
Such a problem can be somewhat improved by increasing the initializing time interval. However, there is a problem in that the luminous efficiency at the first frame is still less than or equal to 50%.
FIG. 3 is a graph showing response time characteristics of the OLED display device shown in FIG. 1 according to different initializing time intervals. That is, FIG. 3 is a graph showing changes in brightness according to initializing time intervals when the screen is changed from black to white.
FIG. 3 shows changes in brightness over time according to the initialization times of 0 μs (a), 1 μs (b) and 2 μs (c). It can be seen that the longer initializing time intervals exhibit better response characteristics. However, it can be seen that the brightness immediately after the screen is changed from black to white (after 0.01 second) is still 50% or less of the normal value in all of the initialization times of (a), (b) and (c).